1. Field of the Invention
The present invention relates to a process for producing a propylene-tetrafluoroethylene copolymer. More particularly, it relates to a novel process for producing the propylene-tetrafluoroethylene copolymer by copolymerizing propylene and tetrafluoroethylene in an aqueous medium containing a specific amount of tertiary butanol and an emulsifier in the presence of a specific redox catalyst to attain smooth copolymerization at high polymerization velocity without discoloring the latex or the copolymer and with high storage stability of the latex.
2. Description of the Prior Art
It has been known that propylene-tetrafluoroethylene copolymers are vulcanizable elastic polymers having excellent heat resistant and chemical resistance.
The copolymerizations of propylene and tetrafluoroethylene in an aqueous medium have been known as disclosed in Japanese Examined patent publication No. 19188/1965, and No. 4115/1978 and U.S. Pat. No. 3,467,635. It has been also proposed that the propylene-tetrafluoroethylene copolymer having high molecular weight can be obtained by copolymerization at a relatively low temperature at 0.degree. to 50.degree. C. if a redox catalyst comprising a water soluble persulfate/a water soluble thiosulfate or sulfite/a water soluble iron salt as a catalyst for the copolymerization in an aqueous medium, as disclosed in British Pat. No. 1,456,615.
On the other hand, many redox catalysts for emulsion polymerization at a lower temperatures have been known for a long time. In many instances the catalyst activity is specifically dependent upon the combination of a given redox catalyst with a given monomer to be polymerized. The limitation for the selection is severer in the case of multi-component copolymerization and is related to various factors such as reactivity of monomer, efficiency for initiation, chain transfer, kinetic chain length, pH of the medium and stabilities of the monomer and the catalyst. Therefore, it is not easy to find an optimum redox catalyst for a specific polymerization system. In the case of the copolymerization system having high chain transfer activity and short kinetic chain length such as a copolymerization of propylene and tetrafluoroethylene, the selection of an optimum redox catalyst is especially difficult. For example, most catalysts having high catalytic activity for the emulsion polymerizations of tetrafluoroethylene at a low temperature are not active for copolymerization of propylene and tetrafluoroethylene at a low temperature.
According to the inventor's studies, the specific redox catalysts disclosed in British Pat. No. 1,456,615 are active for the copolymerization of propylene and tetrafluoroethylene at a low temperature, however, the copolymerizations at a low temperature with such catalyst have the following disadvantages.
Firstly, the copolymerization velocity is not high enough to be upto about 17 g./l.hr. The discoloring (dark-coloring) of the latex and the polymer is remarkably severe even after the complicated step of a long time for the washing because of the use of the activating solution containing a reducing sugar. Moreover, COD of the waste solution from the step of the coaggulation of the latex and the washing is remarkably high and accordingly, it is remarkably disadvantages in view of the treatment of the waste solution. Furthermore, the storage stability of the latex is insufficient with the result being the sedimentation and separation of the latex upon long storage or because the reducing sugar is perishable the latex can become discolored and have a bad smell. The preparation of the activated solution is complicated and the activated solution is immediately inactivated upon contact with oxygen in air. Therefore, the preparation is not easy and the catalytic activity is not stable.